排序方式: 共有34条查询结果,搜索用时 187 毫秒
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Dr. Yifeng Chen David Huang Yizhou Zhao Prof. Dr. Timothy R. Newhouse 《Angewandte Chemie (International ed. in English)》2017,56(28):8258-8262
The telescoping of allyl-palladium catalyzed ketone dehydrogenation with organocuprate conjugate addition chemistry allows for the introduction of aryl, heteroaryl, vinyl, acyl, methyl, and other functionalized alkyl groups chemoselectively to a wide variety of unactivated ketone compounds via their enone counterparts. The compatibility of the dehydrogenation conditions additionally allows for efficient trapping of the intermediate enolate with various electrophiles. The utility of this approach is demonstrated by comparison to several previously reported multistep sequences. 相似文献
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The output signal-to-noise ratio (SNR) of the ultrasound Doppler process, with and without the commonly used sample and hold circuit, is calculated rigorously, and confirmed by experiment. On the basis of this result it is shown that the use of pulsed Doppler processing without increase of the transmitted power enhances the output SNR of a pulse-echo system observing moving particles by the factor frTt, where fr is the repetition rate of the pulse echo and Doppler systems and Tt is the transit time of the particles across the range cell. If nonlinear losses are significant, then changing to continuous Doppler operation and lowering the peak transmitted power to maintain the average power constant can produce additional improvement in SNR due to reduced non-linear losses in the transducer and the propagation medium, as well as reduced requirements for transducer damping. 相似文献
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A split-spectrum processing technique for an ultrasonic flaw detection system has been developed which improves the flaw-to-grain echo ratio in large-grained materials. The enhancement is achieved by partitioning a wide-band received spectrum to obtain frequency shifted bands, which are then processed to suppress the grain echoes with respect to the flaw echo, using a novel signal minimization algorithm. Experimental data for titanium and stainless steel samples are presented which show superior flaw detection capabilities for the minimization algorithm with respect to frequency averaging techniques. 相似文献
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Computer simulations on binary reactions of random walkers (A + A → A) on two- and three-dimensional percolation clusters bear out the recent superuniversality conjecture (integrated reaction rate α t213). Moreover, the fractal-to-euclidean crossover (t213 to t dependence) parallels that of the single walker. 相似文献
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Masha Elkin Anthony C. Scruse Aneta Turlik Timothy R. Newhouse 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(4):1037-1041
A biomimetic cationic structural rearrangement of the oleanolic acid framework is reported for the gram‐scale synthesis and structural reassignment of justicioside E aglycone. The mechanism of the putative biosynthetic rearrangement is investigated with kinetic, computational, and synthetic approaches. The precursor to rearrangement was accessed through two strategic advancements: (1) synthesis of a 1,3‐diketone via oxidation of a β‐silyl enone, and (2) diastereoselective 1,3‐diketone reduction to form a syn‐1,3‐diol using SmI2 with PhSH as a key additive. 相似文献
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The double frequency method, where bubbles rising through a fluid are illuminated simultaneously with a ‘pump’ frequency swept through the resonance frequency range and a megahertz imaging frequency, is used to measure pressure changes in fluids non-invasively. The bubble echo spectrum is centred at the imaging frequency with side bands at separations equal to the pump frequency. If the ambient pressure is changed, the bubbles undergo changes in their sizes. This would alter the resonant frequency of the bubble and hence the peak of the echo at the sum (or difference) frequency. By observing shifts in the frequency of the peak amplitude of the side bands it is shown that fluid pressure changes can be measured accurately. 相似文献